This invention concerns a biaxially oriented and heat set film made from thermoplastic polyester material having improved abrasion resistance.
Films from thermoplastic materials are usually draw oriented in order to improve their mechanical properties; in particular, they are biaxially oriented, whereby the molecular chains of the polymers are oriented in two main directions. In order to impart satisfactory dimensional stability, these films are usually subjected to a heat setting process after the draw process. In this, the films are briefly heated to a temperature just below the melting temperature of the polymers used. Heat setting fixes the polymer chains in the orientation condition caused by the previous drawing. A film which has been heat set in this manner does not have any undesired shrinkage behavior when heated at a later time.
Apart from mechanical properties, additional criteria for thermoplastic films are their surface properties, such as slip properties, roughness, electrostatic charge, scratch resistance and abrasion resistance. There are many publications which disclose additives or coatings to improve sliding behavior and slip. Others describe an antistatic coating for films. Coatings and treatment methods to improve scratch and abrasion resistance are described in DE-OS No. 34 15 835, DE-OS No. 31 01 232 and EP-A- No. 0 074 750.
It has now been discovered that the scratch and abrasion resistance of thermoplastic films, especially of thermoplastic polyester films, is, inter alia, a function of the degree of crystallization of the film surface.
Based on this knowledge, the object of this invention is to develop a film from thermoplastic polyester material which has abrasion resistance superior to the current state of technology, as well as to describe a process for manufacturing such a film.